Base pair bonding in DNA. Molecular graphic of the guanine-cytosine base pair bond in a molecule of DNA (deoxyribonucleic acid). Atoms are depicted as spheres and color-coded: carbon is yellow, hydrogen is white, oxygen is gold, nitrogen is silver/gray. Guanine is seen at left, linked to cytosine (foreground) by three hydrogen bonds. The DNA molecule consists of two long, helical strands held together by 'rungs'. Each rung consists of a pair of nucleotide bases - either guanine and cytosine or thymine and adenine. The order of bases along the rungs of the molecule acts as a code that stores the organism's genetic information in the form of genes.